Rapid detection of weld fatigue crack with anticorrosive coating

1. Introduction

Widely used in industry of pressure vessel, pipes and structures are mostly made from steel,And there are many weld, under the long-term reaction, weld are most vulnerable to injury, the most common defects are produced by fatigue surface crack,  harmfulness of surfacefatigue crack is more harmful  than inside buried crack，The sharp crack tip can cause stress concentration, a serious accident prompted due to rapid extension of crack,As a result, the weld cracks on the lookup and crack depth measuring, has been a problem in the industry. Ultrasound can be used to check the weld, but the probe must be directly coupled to the workpiece surface directly, and complex operation。Usually adopts magnetic powder or infiltration method to find the surface crack, and then by a potential method to measure the crack depth. The above several methods need to be seized cleaning in advance and remove the surface coating, paint layer or dirt, detection efficiency is low, the requirements in a short maintenance period inspection of weld equipment in service, is difficult to accept. Traditional eddy current method can also be used to detect structural steelwork, and do not need to remove surface dirt or paint layer is thinner, but this method is only applicable to check the crack on the parent metal, welding crack in sew is urgent because of the weld during high temperature > great change and ferromagnetic domain of chaotic interference and can not be implemented, rugged, of course, welding and parent metal and the welding filler material difference, is one of the causes to detect problems。

There are a variety of nondestructive testing methods are used to weld crack detection,Magnetic particle (MP), seepage (LP), ultrasonic (UT), X ray (RT) and vortex (ET),Their respective advantages disadvantages compared as follows

Magnetic power is direct ,easy to operate,but not suitable for workpiece with coating or surface is wet 

Ultrasonic method is a good way to find the workpiece surface defects, but also no surface coating, and the test results related to the operator level

Osmosis is suitable for surface crack, but also requires the workpiece surface clean and dry, and sentenced to injury is associated with the operator level

X Ray method is easy to find workpiece surface crack or crack close to surface。Different from other methods,it requires goodpreventive measure, avoid personal injury。

Electromagnetic induction method is well suited for detecting surface crack, and penetrate the surface there are several millimeters thick coating. It not only can be used in damp conditions, even can directly use underwater. Conventional eddy current method is affected by many kinds of interference factors. Judgment is also related to the operator level。This paper introduces a adopt ac electromagnetic field technology and digital technology of weld crack flaw detector, can be in with a thicker coating on the metal surface of a quick scan of weld or the parent metal fatigue crack opening, and canaccurately measure the crack depth values。

2. Detecting principle

Using an alternating magnetic field magnetic steel specimens of local area, when defects into the alternating magnetic field, can produce abnormal leakage magnetic field gradient in the surface area, using magnetic sensor to detect the magnetic field distortion, can get the defect signal, because the specimen is also a good conductor, under the alternating magnetic field induction, specimens induced current will be generated, and at the same time generate additional induced magnetic field and leakage magnetic field of the original synthetic composite magnetic field. Electromagnetic field distortion for ferromagnetic materials, communication, can be strengthened because of the increase of permeability。Because ac electromagnetic field test sensor and coupling of the specimens in the form of an electromagnetic wave transmission, so between them not like ultrasonic method by means of coupling agent, can be directly through the nonconductive coating, rust, even not too thick stainless steel covering layer。

This is quick and accurate to find the method of weld crack,can make qualitative and semi-quantitative evaluative on weld crack.